Is there a source of historical data which assigns a relatively consistent and accurate timestamp to each block? This might take the form of a server having precisely logged when they first heard about each block header. Bonus points if extinct blocks are included.

You probably should state about which period of time you care as more bitcoin-related services emerged as bitcoin gained popularity and what you're asking for requires the existence of a service back in the period of time you care about.
– UTF-8Oct 5 '16 at 12:20